RE Feature
Algae- A source of
Biofuel
The use of algae is an attractive option for generating biofuels. In fact, algae farming
in less than 2-3 per cent of India’s total land could satisfy the country’s liquid fuel
needs in the decades to come.
Renu Singh, Monika Srivastava, Sapna Tiwari
B
iofuels are any solid, liquid or gaseous fuels that are derived from living matter,
either directly from plants or indirectly from agricultural, municipal, commercial
or domestic wastes. First generation biofuels are derived from sugars, starch,
vegetable oils or animal fats. Second generation biofuels are obtained from lignocellulosic biomass. Third generation biofuels or algae biofuels are produced from algae.
Algae need primarily three components to grow: sunlight, carbon-dioxide and water.
They use sunlight to form all kind of substances like sugars, protein, fat, vitamins etc.
and are available in most geographical regions. In principle, a biochemical process
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Some algae species are
known to produce high levels of
carbohydrates instead of lipids as
reserve polymers.
converts the energy of sunlight into a form of chemical energy. Algae needs low input
and gives high yield up to 30-100 times more. Algae like Pleurochrysis carterae, grows
on the fresh or brackish water and thus does not compete with food crops. There is
enough advancement in open ponds and closed photo-bioreactors for productivity and
reliability of algae as a source of biomass for fuel. When physical conditions are stressful
or certain nutrients are lacking, microalgae respond by producing more carbohydrates
or lipids. Some algae species are known to produce high levels of carbohydrates (sugars
and starch) instead of lipids (oils) as reserve polymers. Extensive research on algal biofuel
regarding the algal strains, algae culture, harvesting and downstream process has been
done.
Biodiesel obtained
from algal lipid is
non-toxic and highly
biodegradable.
Microalgae produce
15-300 times more oil
than traditional crops
on an area basis.
Algae cultivation inputs
Algae are simple microorganisms, which range from 0.2 µm in diameter, in picoplankton, to large leaf like formations which are 60 m in length. Algae cultivation for
biofuel production entails carbon dioxide, light, nutrients, optimum temperature and
biomass harvesting.
■ A dominant factor in algal growth is carbon dioxide (CO2) which is used as a carbon
source. No growth occurs in the absence of CO2.
■ Algae need sunlight as an energy source to perform photosynthesis. The rate of
photosynthesis increases with the intensity of light.
■ Algae require nutrients to grow, mainly nitrogen and phosphorous. These can be
supplied in the form of agricultural fertilizers. Waste water effluents also contain a large
amount of nitrogen and phosphorous and are thus useful for algal growth.
■ Algae need optimum temperature for their growth. Water temperature of 20-35ºC is
good for high algal growth, depending on the species.
■ Algae harvesting is very expensive due to its small size and low biomass concentration.
After cultivation, more than 99 per cent by weight of the algae-water mixture is water.
Separation of algae from water demands high energy and thus makes the process costly.
About 20-30 per cent of the total cost of production of biomass is utilized for harvesting.
Different techniques applied for biomass concentration and harvesting are settling ponds,
filtration, centrifugation, flocculation and killing the cells with ultrasound.
Bio-ethanol production
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Microalgae produce fermentable carbohydrates without lignin, which makes them an
Different production processes
attractive feedstock for bio-ethanol production. Algae produce a mixture of methanol, from algae
ethanol, propanol, and butanol. Ethanol can be obtained
from algae by two methods: saccharification and fermentation
Biodiesel
of algae and intracellular fermentation of algae. In the first
TAG
method, the algae which has a good amount of starch (e.g.
ALGAE
Chlorella vulgaris, 37 per cent) is harvested and saccharified
Starch
to convert complex sugar into fermentable sugars by using
Bioethanol
suitable enzymes. Then the simple sugars are subjected to
Biomass A
fermentation in the fermenter under controlled conditions
na
from
ero
d
i.e. pH, temperature etc; 65 per cent ethanol-conversion rate
on ALGAE iges bic
i
t
a
tio
c
n
sifi
can be obtained. Therefore, algal starch proves to be a good
Ga
Biomethane
source of ethanol production using the conventional process.
Syngas
Another type of ethanol production process is intracellular
starch fermentation under dark and anaerobic conditions.
Electricity
Biohyrogen
Algal starch, which is photosynthetically accumulated in
Chlamydomonas was observed to metabolize into low
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Algae- A source of Biofuel
Table 1: Capacity addition in 2012-13 (as on 28th February 2013)
Advantages
High growth rate
High efficiency in CO2 mitigation
Less water demand than land crops
More cost effective farming
Disadvantages
Low biomass concentration
Higher capital costs
Inability to withstand fluctuating temperatures, seasonal change
molecular weight compounds such as hydrogen, ethanol, glycerol and acetate under
oxygen free, CO2 atmosphere. Dark fermentation of green algae Chlorococcum littorale,
which has 27 per cent cellular starch, yields maximum ethanol productivity of 450
umol/g-dry weight at 30ºC.
Biodiesel production
Biodiesel obtained from algal lipids is non-toxic and highly biodegradable. For the same
area microalgae produce 15-300 times more oil for biodiesel production than traditional
crops. Algal biomass contains three main component - proteins, carbohydrates, and
natural oil. About 40 per cent of algal biomass is comprised of fatty acids, which can
be extracted and converted into biodiesel. First, extraction of oil is carried out. Next
the extracted oil is evaporated under vacuum to release solvent mixture solutions using
rotary evaporator at 40-45 °C. Then, the oil produced from each algal species is mixed
with a blend of catalysts such as sodium hydroxide and an alcohol such as methanol.
This process is called trans-esterification. The algal biodiesel can be obtained after
removing glycerol and other valuable products.
Methane production
Methane can be produced from algae through anaerobic digestion or by pyrolysis/
gasification process. It is important as electricity generation can also be used as a vehicle
fuel. It is environment friendly as it generates relatively lesser amount of carbon dioxide
for each unit of heat that is released.
Bio-hydrogen production
The production of hydrogen through biological processes, such as the one using
algae can efficiently solve world’s energy crisis. Algae can produce hydrogen during
photosynthesis and it is possible to optimize hydrogen production by changing algal
Biodiesel production from algae
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surrounding conditions or through genetic manipulation. Biological production of
hydrogen is better than the chemical processes, as biological processes are cost effective.
There are numerous processes for the production of bio-hydrogen such as direct biophotolysis, indirect bio-photolysis, non-photochemical, and deprivation.
Direct bio-photolysis: In this process, hydrogen is produced from water during the
process of algal photosynthesis and sunlight is converted into chemical energy. Photosystem II (PSII) absorbs light energy and generates electrons which are transported
to ferredoxin by using the light energy absorbed by photo-system I (PSI). In the end,
hydrogenase enzyme accepts the electron from the reduced ferredoxin and generates
hydrogen by using the available protons.
2H2O
Light energy
2H2 + O2
Indirect bio-photolysis: Cyanobacteria or blue green algae can also produce hydrogen
through photosynthesis. The process takes place in special structures known as
heterocysts. They use water as an electron donor; oxygen is generated as a byproduct.
Mainly, hydrogenase and nitrogenase enzyme are involved in this process. Hydrogen
production has been assessed in 14 different genera of cyanobacteria under a wide range
of culture conditions.
12H2O + 6CO2
C6H12O6 + 12H2O
Light energy
Light energy
Methane can be
produced from algae
through anaerobic
digestion or by
pyrolysis/ gasification
process. It is
environment friendly
as it generates
relatively lesser
amount of carbon
dioxide for each
unit of heat that is
released.
C6H12O6 + 6O2
12H2 +6CO2
Non-photochemical hydrogen production: In non-photochemical hydrogen
production, starch is converted through glycolysis into pyruvate and NAD+ oxidizes
to NADH and H+. Then the electron is transferred to plastoquinone pool by NAD(P)
H plastoquinone reductase complex and finally photosystem I transfers the electron to
ferredoxin which in turn transfers it to hydrogenases and hydrogen is produced. Under
anaerobic conditions, through pyruvate-ferredoxin oxidoreductase complex, pyruvate
oxidizes to acetyl-CoA and converts oxidized form of ferredoxin to reducing form,
which is useful for hydrogen production.
Sulphur deprivation: Hydrogen is also produced through sulphur deprivation process.
The rate of hydrogen production is enhanced many times by depriving Chlamydomonas
reinhardtii of sulphur. During this process, the rate of oxygen formation and CO2 fixation
decreases significantly due to depletion of D1 in the PSII reaction centre. Deprivation
of sulphur leads to depletion of D1 as D1 polypeptide chain consists of several sulphur
containing amino acids such as cysteine and methionine. The photosynthetic rate
decreases as compared to the mitochondrial respiration rate. Therefore, after sometime
anaerobic condition arises. During partial inactivation of PS II, the electron generated
through photolysis of water is accepted by the protons with the help of Fe-hydrogenase
enzyme, and hydrogen is evolved.
Carbon consumption
Algae use carbon dioxide as a major source of carbon for performing photosynthesis.
CO2 emitted by coal fired power plants, carbon intensive industries and transportation
fuels can be removed by constructing an algae pond or farm. Algae consume emitted
carbon dioxide for their growth and generate a number of biofuels such as bio-ethanol,
biodiesel etc. Biofuels produced have a potential to replace fossil fuels and hence, reduce
carbon emission efficiently. Algae yield greater volumes of biofuel per acre of productionabout 2000 gallons of fuel per acre of production per year, which is better than any crop-
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RE Feature
based biofuel production system such as sugarcane, which yields 450 gallons/
acre/year, palm which yields 650 gallons/acre/year or corn which yields 250/
acre/year. Algal biofuels has evolved as a carbon-neutral source for production
of biofuel. The process will capture carbon dioxide, mitigate green house gas
emission and provide an alternative to fossil fuels.
Challenges
The major challenges of producing biofuel from algae include strain isolation,
nutrient sourcing and utilization, production management, harvesting,
co-product development, fuel extraction, refining and residual biomass
utilization. Technologies for extraction of oil from algae have been successfully
demonstrated but are relatively expensive, in terms of equipment needed and
amount of energy required to extract the oil. Also, there are some limitations
with the handling of algae. The most common and important limitation is the
danger of contamination that enhances the competition with the target species.
If these problems are overcome, the advent of algal biofuel will definitely bring
about development in the field of transport, power generation, and industries.
Environmental issues such as global warming, carbon sequestration and food
security may also be elucidated through algae cultivation. Potential of algal biofuel production in India
In India, a large quantity of waste water is extracted from industries, factories,
municipal sewage etc. Municipal waste water of metro cities contains nutrients
approximately 30-100 ppm of nitrogen, 10-45 ppm of phosphorous - which can
be utilized for algal cultivation. Only, a small quantity of agricultural wastewater
is recycled on farms, while most of it is discharged. This discharged water can
be used for algal growth. All these techniques not only fulfill the nutrient needs
of algae at a very cheap rate, but also provide a solution for treating wastewater.
Large scale micro-algae cultivation can be done in coastal areas and in flat plain
regions. Also, the Indian climate is suitable for algal growth. According to an
estimate, algae farming in less than 2-3 per cent of India’s total land would
make the country self sufficient in liquid fuel. Although land, water and
nutrient sources are easily available
for cultivation of algae in India, but,
its adaptation on a commercial scale
would require development of new
technologies. Harvested algae should
be quickly extracted for algal crude
oil. It should be compatible with the
current engines. Technologies related
to processing of algal crude into
bio-methane or biodiesel should be
developed at village level. b
Biological production of hydrogen is better
than the chemical processes as biological
processes are cost effective.
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The authors are Senior Scientist, Scientist,
and Senior Research Fellow at Centre for
Environment Science and Climate Resilient
Agriculture, Indian Agricultural Research
Institute, Pusa.
Email- [email protected]
According to an
estimate, algae
farming in less than
2-3 per cent of India’s
total land would make
the country
self sufficient in
liquid fuel.